Abstract

The conventional K-Ar dating method was applied to the 1986 dacite flow from the new lava dome at Mount St. Helens, Washington. Porphyritic dacite which solidified on the surface of the lava dome in 1986 gives a whole rock K-Ar “age” of 0.35±0.05 million years (Ma). Mineral concentrates from the dacite which formed in 1986 give K-Ar “ages” from 0.34±0.06 Ma (feldspar-glass concentrate) to 2.8±0.6 Ma (pyroxene concentrate). These “ages” are, of course, preposterous. The fundamental dating assumption (“no radiogenic argon was present when the rock formed”) is questioned by these data. Instead, data from this Mount St. Helens dacite argue that significant “excess argon” was present when the lava solidified in 1986. Phenocrysts of orthopyroxene, hornblende, and plagioclase are interpreted to have occluded argon within their mineral structures deep in the magma chamber and to have retained this argon after emplacement and solidification of the dacite. The amount of argon occluded is probably a function of the argon pressure when mineral crystallization occurred at depth and/or the tightness of the mineral structure. Orthopyroxene retains the most argon, followed by hornblende, and finally, plagioclase. The lava dome at Mount St. Helens dates very much older than its true age because phenocryst minerals inherit argon from the magma. The study of this Mount St. Helens dacite causes the more fundamental question to be asked—how accurate are K-Ar “ages” from the many other phenocryst-containing lava flows world-wide?